Bells rang across New York and moments of silence were observed today in remembrance of the 2,792 people killed at the World Trade Centre on September 11 two years ago, when hijacked planes destroyed the 110-story twin towers.
As the ceremony marking the anniversary took place at Ground Zero, the State Department in Washington urged Americans overseas to take special caution, citing growing indications the Islamist al Qaeda network, blamed for the September 11 attacks, was planning even "more devastating" attacks.
In New York thousands of victims' relatives, some holding flowers and portraits of their loved ones, took part in an almost four-hour long solemn ceremony. Some wore T-shirts showing portraits of the dead, police and firefighters wore dress uniforms, bagpipes were played and many were tearful.
The children of those who perished read out the names of the dead, leaving many family members wracked with tears.
The children reading the names stood at a dais two by two, some wearing suits but most dressed casually, and read out the names, sometimes stumbling over pronunciations, each sequence ending with a child reading out the name of their dead mother, father, uncle, bother or other relation.
New York Mayor Michael Bloomberg began the ceremony saying that the children carried "both our deepest memories and the bright promise of tomorrow."
Bells rang and silence was observed at 8.46am (1.46pm Irish time) to mark the moment when the first plane hit one tower and at 9.03am, when the second hijacked airliner crashed into the other tower and again at 9.59am and 10.29am when each tower crumbled.
In Washington, President George W. Bush attended a church service to remember the victims of the New York attacks and of the simultaneous crash of two other hijacked planes - one into the Pentagon and the other into a field in Pennsylvania.
Relatives walked to the bottom of the 16-acre site to lay flowers where the World Trade Centre once stood, now a vast construction site and the city's open wound from the attacks, blamed on the Osama bin Laden's al Qaeda network.